Effect of Bromide and Iodide Ions on the Formation and Speciation of Disinfection Byproducts during Chlorination

TLDR

The study examined how adding bromide or iodide ions (0–30 µM) to natural waters and chlorinating them affects the formation and speciation of disinfection byproducts. The authors measured trihalomethanes, haloacetic acids, total organic halogen, and its halogen‑specific fractions (TOCl, TOBr, TOI) in the chlorinated, bromide‑ or iodide‑fortified waters. Adding bromide increased THM and HAA yields but left total organic halogen unchanged, whereas iodide addition lowered TOX through oxidation to iodate, leading to fewer iodinated byproducts even at higher chlorine doses; these trends agreed with published kinetic data.

Abstract

Two natural waters were fortified with various levels of bromide or iodide ions (0−30 μM) and chlorinated in the laboratory to study the impact of bromide and iodide ions on the formation and speciation of disinfection byproducts. Trihalomethanes (THMs), haloacetic acids (HAAs), total organic halogen (TOX), and its halogen-specific fractions total organic chlorine (TOCl), bromine (TOBr), and iodine (TOI), were measured in this work. The molar yields of THMs and HAAs increased as the initial bromide concentration increased. No significant change in TOX concentration was found for varying bromide concentrations. However, TOX concentrations decreased substantially with increasing initial iodide concentrations. At higher levels of bromide, there was a decreasing level of unknown TOX and unknown TOCl but an increasing level of unknown TOBr. The extent of iodine substitution was much lower than that of bromine substitution when comparing identical initial concentrations because a substantial amount of iodide was oxidized to iodate by chlorine. The tendency toward iodate formation resulted in the unusual situation where higher chlorine doses actually caused reduced levels of iodinated organic byproducts. Quantitative assessment of the results of this study showed a good agreement with kinetic data in the literature.